1. Field of the Invention
The present invention relates to a light guiding unit, and a light illuminating device and an image reading apparatus using the same. In particular, the present invention is suitable for an image reading apparatus configured to illuminate an original surface to read an image in a line sequential system, such as an image scanner, a copying machine, and a fax machine.
2. Description of the Related Art
Hitherto, there has been known a technology of using a light emitting diode (LED) as a light source for an original illuminating device that is a light illuminating device in an image reading apparatus. The light illuminating device may be classified into an array arrangement-type light illuminating device in which a plurality of LEDs are arranged in a main scanning direction, and an end portion arrangement-type light illuminating device in which an LED is arranged at a longitudinal end portion of a translucent light guiding member made of a resin or the like, to thereby propagate light fluxes emitted from the light source through the light guiding member.
Recently, the luminous efficiency of the LED is enhanced, and hence there is a demand for the end portion arrangement-type light illuminating device in which the usage number of LEDs can be reduced. The end portion arrangement type has a problem in that, as compared to the array arrangement type, the illuminance of a part of a region to be illuminated on an original reading surface, which is close to an end surface into which light from the LED enters (incident surface), is less liable to be enhanced. This problem attributes to an optical path length of a light beam emitted from the LED to enter the light guiding member, which is reflected by a diffusion surface formed in a longitudinal direction (main scanning direction) of the light guiding member to pass through a light guiding member exit surface and reach an original surface, and attributes to an angle of the light beam reflected at the diffusion surface.
In particular, when the diffusion surface has a saw-like triangular prism or trapezoidal prism shape, the angle of the light beam traveling toward the original surface tends to be directed in a direction in which the light beam is separated from the end surface (incident surface), and hence the illuminance near the end surface (incident surface) has been less liable to be enhanced.
Further, when the object to be read has a large thickness, there has been a problem in that the shadow of the three-dimensional object tends to be generated only in one direction of the longitudinal direction (main scanning direction). That is, similarly to the problem in that the illuminance near the end surface (incident surface) is less liable to be enhanced, the angle of the light beam traveling toward the original surface tends to be directed in the direction in which the light beam is separated from the end surface (incident surface), and hence there has been a problem in that the shadow is generated due to the thickness of the object to be read.
As a proposal for addressing such problems, in Japanese Patent Application Laid-Open No. 2011-71696, there is disclosed a technology for improving the main scanning illuminance by extending the light guiding member in the main scanning direction so that the light beam totally reflected by the diffusion surface (saw-like triangular prism shape) of the light guiding member illuminates a predetermined illuminance region.
Further, in Japanese Patent Application Laid-Open No. 2009-37746, there is disclosed a technology for suppressing the shadow of the three-dimensional object by forming a surface of a reflective plate, which is arranged to be opposed to the diffusion surface (saw-like triangular prism shape) of the light guiding member, into a concavo-convex shape.
A downsized image reading apparatus has a high need. In contrast to this demand, in Japanese Patent Application Laid-Open No. 2011-71696, the light guiding member is extended in the main scanning direction, which is unsuitable for downsizing. Particularly in the end portion arrangement type, members such as the light guiding member, the LED light source, and a radiator member are arranged in the longitudinal direction (main scanning direction) of the light guiding member, and hence extending the light guiding member in its longitudinal direction (main scanning direction) is closely associated with upsizing of the apparatus.
Further, in Japanese Patent Application Laid-Open No. 2009-37746, the surface of the reflective plate is formed into a concavo-convex shape, but when the relative position between the pitch or shape of the diffusion surface of the light guiding member and the pitch or shape of the concavo-convex shape of the reflective plate is misaligned in the longitudinal direction (main scanning direction), the shadow of the three-dimensional object may be generated. Therefore, in addition to the requirement of the high positional accuracy between the light guiding member and the reflective plate, there is a fear in that the pitch or shape of the diffusion surface of the light guiding member may be shifted due to thermal expansion caused by the heat generated by the LED light source.